[flang] Fix character initialization after continuation

The insertion of a space on a line continuation right before
a character literal was confusing TokenSequence::ToLowerCase(),
that was unable to identify the character literal as such,
causing it to be converted to lower case.

Fix this by skipping spaces in the beginning and end of each
token, before testing for token type.

Fixes #62039

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D151885

flang/lib/Parser/token-sequence.cpp

@@ -155,7 +155,16 @@ TokenSequence &TokenSequence::ToLowerCase() {
     std::size_t nextStart{atToken + 1 < tokens ? start_[++atToken] : chars};
     char *p{&char_[j]};
     char const *limit{char_.data() + nextStart};
+    const char *lastChar{limit - 1};
     j = nextStart;
+    // Skip leading whitespaces
+    while (p < limit - 1 && *p == ' ') {
+      ++p;
+    }
+    // Find last non-whitespace char
+    while (lastChar > p + 1 && *lastChar == ' ') {
+      --lastChar;
+    }
     if (IsDecimalDigit(*p)) {
       while (p < limit && IsDecimalDigit(*p)) {
         ++p;
@@ -172,17 +181,17 @@ TokenSequence &TokenSequence::ToLowerCase() {
           *p = ToLowerCaseLetter(*p);
         }
       }
-    } else if (limit[-1] == '\'' || limit[-1] == '"') {
-      if (*p == limit[-1]) {
+    } else if (*lastChar == '\'' || *lastChar == '"') {
+      if (*p == *lastChar) {
         // Character literal without prefix
-      } else if (p[1] == limit[-1]) {
+      } else if (p[1] == *lastChar) {
         // BOZX-prefixed constant
         for (; p < limit; ++p) {
           *p = ToLowerCaseLetter(*p);
         }
       } else {
         // Literal with kind-param prefix name (e.g., K_"ABC").
-        for (; *p != limit[-1]; ++p) {
+        for (; *p != *lastChar; ++p) {
           *p = ToLowerCaseLetter(*p);
         }
       }

flang/test/Parser/continuation-before-char.f90

@@ -0,0 +1,7 @@
+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! Continuation right before character literal.
+subroutine test()
+! CHECK: CHARACTER(LEN=3_4) :: a = "ABC"
+  character(len=3) :: a =&
+"ABC"
+end subroutine