fix usage of "empty" libraries

a header-only library in a default location would produce no variables
at all, making it appear undefined. fix this by forcing the writeout of
the QMAKE_LIBS_* variable, and use its definedness (rather than
non-emptiness) as a signifier.

this works for both QMAKE_USE and configure tests'/libraries' 'use'
entries.

Change-Id: Id7a1e23725caba1a91ea4db448b4aeb7fe632393
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

4 files changed, 13 insertions, 21 deletions

diff --git a/mkspecs/features/qmake_use.prf b/mkspecs/features/qmake_use.prf
index 9b6c67ccc2..f81d1cece4 100644
--- a/mkspecs/features/qmake_use.prf
+++ b/mkspecs/features/qmake_use.prf
@@ -5,25 +5,19 @@ for(ever) {
         use = $$split(use, /)
         name = $$take_first(use)
         nu = $$upper($$name)
-        debug: \
-            libs = $$eval(QMAKE_LIBS_$${nu}_DEBUG)
-        else: \
-            libs = $$eval(QMAKE_LIBS_$${nu}_RELEASE)
-        libs += $$eval(QMAKE_LIBS_$$nu)
-        libdir = $$eval(QMAKE_LIBDIR_$$nu)
-        defines = $$eval(QMAKE_DEFINES_$${nu})
-        includes = $$eval(QMAKE_INCDIR_$${nu})
-
-        isEmpty(libs):isEmpty(defines):isEmpty(includes): \
+        !defined(QMAKE_LIBS_$$nu, var): \
             error("Library '$$name' is not defined.")
 
         !contains(use, nolink) {
-            QMAKE_LIBDIR += $$libdir
-            LIBS$${suffix} += $$libs
+            QMAKE_LIBDIR += $$eval(QMAKE_LIBDIR_$$nu)
+            debug: \
+                LIBS$${suffix} += $$eval(QMAKE_LIBS_$${nu}_DEBUG) $$eval(QMAKE_LIBS_$$nu)
+            else: \
+                LIBS$${suffix} += $$eval(QMAKE_LIBS_$${nu}_RELEASE) $$eval(QMAKE_LIBS_$$nu)
         }
         !contains(use, linkonly) {
-            DEFINES += $$defines
-            INCLUDEPATH += $$includes
+            DEFINES += $$eval(QMAKE_DEFINES_$${nu})
+            INCLUDEPATH += $$eval(QMAKE_INCDIR_$${nu})
         }
     }
     !isEmpty(suffix): break()
diff --git a/mkspecs/features/qt_configure.prf b/mkspecs/features/qt_configure.prf
index 69203abf07..b1c8d5b33a 100644
--- a/mkspecs/features/qt_configure.prf
+++ b/mkspecs/features/qt_configure.prf
@@ -603,7 +603,8 @@ defineTest(qtConfExportLibrary) {
         qtConfAddNote("Dropped compiler flags '$$ignored' when detecting library '$$name'.")
 
     NAME = $$upper($$name)
-    !isEmpty(libs): qtConfOutputVar(assign, $$output, QMAKE_LIBS_$$NAME, $$libs)
+    # LIBS is emitted even if empty, as this allows the library to be "seen".
+    qtConfOutputVar(assign, $$output, QMAKE_LIBS_$$NAME, $$libs)
     for (b, $${spfx}.builds._KEYS_): \
         qtConfOutputVar(assign, $$output, QMAKE_LIBS_$${NAME}_$$upper($$b), \
                                           $$eval($${spfx}.builds.$${b}))
@@ -707,11 +708,7 @@ defineTest(qtConfTestPrepare_compile) {
         }
         isEmpty(libConfig) {
             nu = $$upper($$u)
-            libs = $$eval(QMAKE_LIBS_$$nu) $$eval(QMAKE_LIBS_$${nu}_DEBUG) $$eval(QMAKE_LIBS_$${nu}_RELEASE)
-            defines = $$eval(QMAKE_DEFINES_$${nu})
-            includes = $$eval(QMAKE_INCDIR_$${nu})
-
-            isEmpty(libs):isEmpty(defines):isEmpty(includes): \
+            !defined(QMAKE_LIBS_$$nu, var): \
                 error("Test $$1 tries to use undeclared library '$$u'")
             $${1}.literal_args += $$system_quote(QMAKE_USE += $$u)
         } else {
diff --git a/mkspecs/features/qt_helper_lib.prf b/mkspecs/features/qt_helper_lib.prf
index 99ba0c6877..b3e49e4626 100644
--- a/mkspecs/features/qt_helper_lib.prf
+++ b/mkspecs/features/qt_helper_lib.prf
@@ -42,6 +42,7 @@ THE_TARGET = $$qt5LibraryTarget($$TARGET)
         darwin: MODULE_DEBUG_LIBS = -L$$DESTDIR -l$${TARGET}_debug
         MODULE_RELEASE_LIBS = -L$$DESTDIR -l$$TARGET
         MODULE_PRI_CONT += \
+            "QMAKE_LIBS_$${ucmodule} =" \  # Needed for the module to be recognized.
             "QMAKE_LIBS_$${ucmodule}_DEBUG = $$val_escape(MODULE_DEBUG_LIBS)" \
             "QMAKE_LIBS_$${ucmodule}_RELEASE = $$val_escape(MODULE_RELEASE_LIBS)"
     } else {
diff --git a/mkspecs/features/qt_module_pris.prf b/mkspecs/features/qt_module_pris.prf
index 57160d11eb..bcaac230ec 100644
--- a/mkspecs/features/qt_module_pris.prf
+++ b/mkspecs/features/qt_module_pris.prf
@@ -59,7 +59,7 @@ defineReplace(qtExportLibsForModule) {
             QMAKE_LIBS_$$NAME QMAKE_LIBS_$${NAME}_DEBUG QMAKE_LIBS_$${NAME}_RELEASE \
             QMAKE_DEFINES_$$NAME QMAKE_INCDIR_$$NAME
         for (var, vars) {
-            !isEmpty($$var): \
+            defined($$var, var): \
                 result += "$$var = $$val_escape($$var)"
         }
     }